**A person can make 100 using the digit 9 four times in an equation.** The equation for this math problem is: 99 + (9 / 9) = 100. This solution works because 9 divided by 9 equals 1, and 99 plus 1 equals 100.

Solving this math problem requires one to notice that the number 99, which contains the digit 9 repeated twice, is very close to 100. In fact, 100 - 99 = 1. The other two 9 digits must therefore equal one, and this requires a fraction. The fraction 9 over 9 equals 1. Combining these two facts in a single equation solves this math problem, with the answer being 99 + (9 / 9) = 100.